1. Field of the Invention
The present invention relates to a voltage detection circuit, and more particularly to a low-voltage detection circuit capable of precisely measuring a voltage.
2. Description of the Related Art
With the advance of technology, hand-held electronic apparatuses, such as electronic dictionaries, cellular phones, walkmans, or digital cameras, which are driven by low voltages, have become commonplace in our daily lives. The power of integrated circuits of the electronic apparatus is supplied by batteries. However, the power capacity of batteries is limited. Therefore, users must pay attention as to when the power is running out and when the battery needs to be replaced or charged. For that purpose, a low-voltage detection circuit is required.
FIG. 1 is a circuit showing a conventional low-voltage detection apparatus. Referring to FIG. 1, the conventional low-voltage detection apparatus comprises a reference voltage source 101, a bias voltage circuit 105, and a comparator 102. The reference voltage source 101 is a band-gap reference apparatus. The apparatus has a low temperature coefficient. That is, the output reference voltage Vref does not vary with the temperature. The bias circuit 105 comprises resistors R111 and R112, and divides the terminal voltage Vdd, which is coupled to the power supply terminal, into the voltage Vdiv. The voltage Vdiv, therefore, is Vdd×R112/(R111+R112).
The comparator 102 receives and compares the divided voltage Vdiv of the terminal voltage Vdd, and the reference voltage Vref between the negative input terminal and the positive input terminal. When the divided voltage Vdiv is larger than the reference voltage Vref, the voltage Vdd should be larger than, or equal to the ideal threshold voltage of the to-be-tested integrated circuit (not shown). As a result, the to-be-tested integrated circuit (not shown) is enabled. The comparator 102 then outputs the voltage detection signal 130 with logic 0 such that the to-be-tested integrated circuit (not shown) starts operation.
On the contrary, when the divided voltage Vdiv is smaller than the reference voltage Vref, the voltage Vdd should be smaller than the ideal threshold voltage of the to-be-tested integrated circuit (not shown). As a result, the to-be-tested integrated circuit (not shown) cannot stably operate under such voltage. The comparator 102 then outputs the voltage detection signal 130 with logic 1 to turn off the to-be-tested integrated circuit (not shown), or to inform the user to change the battery.
The conventional low-voltage detection circuit, however, has two disadvantages:
1. The band-gap reference apparatus of the low-voltage detection circuit must operate with a reference voltage source higher than 1.25 V. If the operational voltage is equal to, or smaller than 1.24 V, the circuit cannot normally operate. Accordingly, the conventional low-voltage detection circuit cannot operate under the voltage lower than 1.24 V.
2. Present integrated circuits are formed by the metal-oxide-semiconductor (MOS). The operation of MOS transistor will be affected by the variation of the process or with different manufacturing dates. Integrated circuits which are formed by different processes and with different manufacturing dates would require different operational voltages and have different threshold voltages. The reference voltage Vref uses the threshold voltage of the worst integrated circuit as a standard.
For example, the operational voltage of a MOS-made integrated circuit is between 2.4 V to 5 V. However, in the integrated circuits formed in the same manufacturing process, but in different dates, the operational voltage of the best integrated circuit is between 2.0 V to 5 V, but the operational voltage of the worst integrated circuit is between 2.8 V and 5 V.
In order for these integrated circuits to operate in a stable condition, the ideal terminal voltage Vdd for each integrated circuit should be higher than 2.8 V. The reference voltage Vref will also vary with the minimum terminal voltage Vdd. The best integrated circuit would warn the user of changing the battery and the integrated circuit would stop operation, when the terminal voltage is lower than 2.8 V, rather than when the terminal voltage is lower than 2.0 V. Accordingly, 0.8 V is wasted.